Control rod for nuclear reactor

ABSTRACT

The invention relates to a control rod for a boiling water reactor. The control rod comprises a top piece and a bottom piece between which an absorber part is arranged. The absorber part comprises a central part ( 4 ) and four absorber blades ( 3 ) extending from the central part ( 4 ). The absorber part comprises a channel ( 10 ) which comprises the central part and which extends along the length of the whole absorber part. The channel is surrounded by walls of a neutron-absorbing material with a long service life, for example hafnium. The channel is at least partially filled with a moderator, for example light water. During the manufacture the absorber part has undergone a heat treatment such that the hafnium alloy has been completely or partially transformed from α-phase to β-phase and thereafter been rapidly cooled to α-phase.

TECHNICAL FIELD

The present invention relates to a control rod for a boiling waterreactor (BWR). More particularly, it relates to a control rod of aneutron-absorbing material which encloses a moderating substance.

BACKGROUND OF THE INVENTION

A core in a nuclear reactor normally contains several hundred fuelassemblies. These are arranged vertically in the core and have an atleast substantially square cross section. Each fuel assembly comprises abundle of fuel rods which are surrounded by a fuel channel. The fuelchannel is open at both ends allowing the coolant of the reactor to flowthrough the fuel assembly. The core is immersed into water which servesboth as coolant and as neutron moderator.

The core also includes a large number of control rods, each of whichcomprises four vertically arranged absorber blades forming aright-angled cross. The fuel assemblies are normally arranged in aregular lattice. Each fuel assembly is included in two rows of fuelassemblies which are perpendicular to each other. The control rods arenormally arranged with each one of their absorber blades between twofuel assemblies located in the same row such that each control rodtogether with four fuel assemblies, arranged around the blades of thecontrol rod, forms one unit. By moving the control rods into and out ofthe core, the power distribution of the core can be controlled.

Conventional control rods for boiling water reactors are composed offour elongated absorber blades which are arranged in a cruciform. Theabsorber blades are generally of stainless which are steel and providedwith a large number of bored holes or tubes filled with a powder ofboron carbide (B₄C), acting as neutron absorber.

When a control rod is inserted into the core in the reactor, the neutronabsorber is irradiated by neutrons and successively loses neutronabsorption capacity. The control rod is therefore replaced after beingused for a predetermined operating period. It is true that boron carbidehas a great ability to absorb neutrons, but it has disadvantages. Sincethe boron carbide is in a powdered state, it must be encapsulated intothe absorber blades. When the boron carbide absorbs neutrons, it swells,which causes great mechanical stresses on the cladding. Control rodswith boron carbide as absorber material therefore have a relativelyshort service life.

A number of different proposals for new control rods with longer servicelives have been produced, one example being disclosed in the patentspecification U.S. Pat. No. 5,034,185. Common to these proposals is thatthe neutron absorber consists of a material with a comparatively longlife, for example hafnium. A further advantage with hafnium, in relationto the boron carbide is its good corrosion properties, which means thathafnium withstands contact with the cooling water and need not beencapsulated. On the other hand, hafnium is not as good a neutronabsorber as boron carbide, so in order to obtain an equally effectivecontrol rod the quantity of absorber material needs to be increased. Oneproblem with hafnium is that it is expensive and has a high density,which entails an increase in both the weight and the cost of controlrods manufactured of this material.

The probability that a neutron will be absorbed increases the lower itsenergy is. Thus, a fast neutron has a very small probability of beingabsorbed. In the patent specification U.S. Pat. No. 4,876,060 a controlrod is described in which the absorber blades comprise absorber platesof hafnium, which are arranged such that a channel filled with water isformed between them. The water moderates (slows down) the neutrons whichare so fast that they are not absorbed the first time they pass throughthe absorber plate. The fast neutrons are slowed down by collisions withthe hydrogen atoms of the water and will be absorbed with a higherprobability the next time they pass through one of the absorber plates.By using a moderator between the absorber plates, the absorptioncapacity of the control rod increases, which means that the absorberplates can be made thinner. In this way, the consumption of theexpensive and heavy absorber material can be reduced. This technique iscommon knowledge and usually referred to as a neutron trap.

The control rod described above is composed of four absorber blades,which are arranged in a cruciform and fixed to a central supporting rod,a so-called center rod. The control rod also has an external shroud of,for example, stainless steel. The center rod consists of a rod whoselength corresponds to the length of the absorber blades in thelongitudinal direction of the control rod. A disadvantage of the controlrod described above is that the volume of the moderator is limited bythe centre rod and the shroud. With a different embodiment of thecontrol rod, the volume of the moderator can be increased and theabsorption capacity of the control rod be further improved.

It is an object of the invention to propose a control rod which

has a high neutron absorption capacity,

has a long service life,

is simple and inexpensive to manufacture,

does not weigh too much.

SUMMARY OF THE INVENTION

The invention relates to a control rod comprising a top piece and abottom piece between which an absorber part is arranged. The absorberpart comprises a central part and four absorber blades extending fromthe central part. The absorber part comprises a channel which comprisesthe central part and which extends along the length of the wholeabsorber part. The channel is surrounded by walls which comprise aneutron-absorbing material with a long service life, for examplehafnium. The channel is at least partially filled with a moderator, forexample light water.

By arranging a moderator also in the central part of the control rod, anincrease of the total volume of the moderator and a large coherentmoderator volume are obtained. A large coherent moderator volume slowsdown the neutrons more than a plurality of smaller moderator volumes.This is due to the fact that the neutrons are able to move a longerdistance in the larger volume and can, therefore, be slowed down to agreater extent. This means a great improvement of the efficiency of thecontrol rod compared with previous solutions where moderator water isonly present in a number of individual channels in the absorber bladesand where the center is occupied by a central rod.

The control rod is simple in construction and is therefore inexpensiveto manufacture. The weight and cost of the control rod can be weighedagainst its service life and its efficiency and is determined in aflexible way by the thickness of the absorber plate.

The control rod becomes flexible in a direction perpendicular to thelongitudinal direction of the control rod. This provides good propertiesfrom a seismic point of view, and the flexibility also prevents thecontrol rod from being stuck in the core, because of bowed control rodsor fuel assemblies or other similar reasons, when it is to be moved outof and into the core.

During manufacture of the absorber part it may undergo a heat treatmentsuch that the hafnium alloy has completely or partially been transformedfrom α-phase to β-phase and thereafter rapidly cooled to α-phase.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an embodiment of a control rod according tothe invention.

FIG. 2 shows the same control rod in a section corresponding to A—A inFIG. 1.

FIG. 3 shows how the channel walls are joined together in a sectioncorresponding to B—B in FIG. 2.

FIG. 4 shows an alternative embodiment of a control rod according to theinvention.

FIGS. 5a-5 f show alternative embodiments of an attachment between twohafnium sheets.

FIGS. 6 to 10 a-d show alternative embodiments of a control rodaccording to the invention. FIGS. 9a and 9 b show a sectioncorresponding to C—C in FIG. 8.

FIGS. 11a and 11 b show alternative embodiments of an attachment betweenthe absorber part and the top piece.

FIGS. 12a-12 c show in a section corresponding to D—D in FIG. 11bdifferent alternatives of the construction of an attachment by means ofa rivet joint.

FIGS. 13 and 14a and b show alternative embodiments of the attachmentbetween the absorber part and the bottom piece.

FIG. 14b show a section corresponding to E—E in FIG. 14a.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 show a preferred embodiment of a control rod according tothe invention. The control rod comprises a top piece 1 and a bottompiece 2 for fixing an absorber part 11 extending therebetween. The toppiece comprises a handle 1 a and distance buttons 1 b, which ensure thatthe control rod, when being inserted into the reactor core, is kept at adefinite distance from the fuel. The absorber part comprises a centralpart 4 and four absorber blades 3 extending from the central part. Thecentral part and the absorber blades together form a cruciform channel10. The absorber part comprises four plates 5 of thin hafnium sheet witha substantially L-shaped cross section in the longitudinal direction ofthe control rod, that is, a plate which is bent through 90° such thateach side of the bending line is equally long. The plates are arrangedfacing each other so as to form a right-angled cross with four absorberblades 3.

For joining the walls of the channel together and keeping them at adistance from each other in the absorber blades, each one of theopposite walls is provided with indentations 7 intermittently arrangedin the longitudinal direction. Each indentation in one wall is directedagainst and welded to a corresponding indentation belonging to the otherwall. In this way, the walls of the channel are joined togetherintermittently along the length of the whole channel. FIG. 3 shows indetail the appearance of the joined indentations in a section B—B inFIG. 2. The control rod is provided with two rows of indentations ineach absorber blade.

The plates are not joined together along the outer edge of the absorberblades. An opening 6 has been left between the plates, which gives agood exchange of water between the inside and outside of the controlrod. The opening 6 extends along the length of the whole absorber part.The water in the control rod causes a slowing down of the fast neutronssuch that they can be more easily absorbed by the hafnium sheet. Duringthe slowing down, heat is released. Neutron absorption in the hafniumsheets and the radiation absorption also contribute to a significantheat generation in the control rod. For the control rod to functionsatisfactorily, it must have thorough cooling. The water used asmoderator also serves as coolant in the control rod. The openings 6 inthe outer ends of the absorber blades provide a flow of cooling waterand, in addition, eliminate the risk of pressure increase in the controlrod. Steam formation in the control rod would also be a safety hazard asit would reduce the effectiveness of the control rod.

The cruciform channel formed from the joined L-shaped plates is open atboth top and bottom such that the cooling water which flows upwardsthrough the core can pass through the control rod. The top piece and thebottom piece are fixed to the absorber part by rivets. Openings 11 a arearranged between the absorber part and the top piece. Between theabsorber part and the bottom part, openings 11 b are arranged. The taskof these openings is to improve the flow of water through the absorberpart. The openings shall be of such a size to create a thoroughcirculation of the cooling water.

In an embodiment of a control rod according to the invention shown inFIG. 4, the control rod only has one row of indentations in eachabsorber blade, and instead the outer ends of the absorber blades arejoined together. The joining of the plates in the outer ends of theabsorber blades can be made by means of, for example, spot welding orrivets. An all-welded edge is unsuitable since stresses are thus builtinto the control rod. In further embodiments the outer end of theabsorber blade may be partially open and partially joined together.

Instead of using indentations as spacers, an inwardly facing fold may bearranged on each wall in the longitudinal direction thereof. Twoopposite side walls are joined by means of, for example, spot weldingalong the folds. The folds divide the large cruciform channel into fivesmaller channels, which consist of a central cruciform channel and fourchannels located in the absorber blades.

FIGS. 5a-5 f show different possible attachments between two hafniumsheets. FIG. 5a shows a spot weld and FIG. 5b shows a fusion weld, forexample a TIG weld. Welding has the advantage that it does not add anysmall parts to the construction which may subsequently loosen and causeproblems. A disadvantage with welding is that it requires expensive andcomplicated welding equipment. FIG. 5c shows a conventional rivet joint,and FIG. 5d shows a rivet joint with a safety device which prevents thewhole rivet from loosening if the rivet breaks. FIG. 5e shows a two-partrivet joint which is welded or threaded and weld-locked. FIG. 5f showsan upset rivet joint where the plates are pressed together in such a wayas to lock against each other. Rivets may be of a material other thanhafnium, for example titanium, zirconium, stainless steel or Inconel(nickel-base alloy). The advantage of a rivet joint is that it is simpleand inexpensive to manufacture. The disadvantage is that it comprises alarge number of small rivets which may possibly loosen after some timein operation.

FIG. 6 shows a further embodiment of the invention, which comprises atube of hafnium on which are arranged four absorber blades 3 of hafniumsheet. The hafnium tube constitutes a centrally located channel 10. Thechannel has a substantially circular cross section and is at leastpartially filled with a moderator. Each absorber blade constitutes achannel which is at least partially filled with a moderator.

The neutron irradiation of the control rod does not take place uniformlyacross the whole surface; for example, an outer portion of the absorberblades is exposed to a higher irradiation than other portions. The upperpart of the control rod, that is, that part which is nearest the toppiece, is also exposed to a higher irradiation than the lower part, thatis, that part which is nearest the bottom piece. Those parts of theneutron absorber which are exposed to a higher irradiation are burnt outfaster than the remainder of the control rod. To obtain an optimumhafnium use, the thickness of the sheet should vary, that is, thickersheets in those parts which are burnt out fastest. However, the strengthof the control rod must also be taken into consideration.

In an alternative embodiment of the invention, a bar ofneutron-absorbing material, for example hafnium, is arranged at theextreme end of each absorber blade in order to increase the amount ofneutron-absorbing material at particularly exposed points. This bar mayalso serve as a base for attachment of the bottom piece and the toppiece. FIG. 7 shows a section through a control rod according to theinvention with a bar 8 inserted between two adjacent plates 5 whichtogether, with the bar, form a control rod blade.

It is desirable to be able to vary the absorption capacity, that is, theamount of hafnium, both axially and radially in the absorber part. FIG.8 shows a control rod according to the invention where the amount ofhafnium varies both axially and radially in the absorber part. In theregions marked 20, the hafnium thickness is greater than in the otherparts of the absorber part. One way of achieving the desired variationin the absorption capacity is to vary the sheet thickness in theabsorber part. FIG. 9a shows in a section C—C in FIG. 8 how the sheetthickness varies radially in an absorber blade. The disadvantages ofthis method are the higher manufacturing costs.

FIG. 9b shows in a section C—C in FIG. 8 another method of varying theabsorption capacity, which means that a hafnium sheet 21 is suspendedloosely in the control rod blade. The hafnium sheet may be varied inlength, width and thickness and may be provided with different holes andrecesses. The attachment of the hafnium sheet may be made in connectionwith the attachment of the absorber blade to the top piece. To preventthe hafnium sheet from possibly becoming detached from the top piece, itmay be secured with the aid of the intermittently arranged indentationswhich keep the control rod blade together. These indentations are notshown in FIGS. 9a and 9 b.

One further way of varying the absorption capacity axially and radiallyin the control rod, which is shown in FIGS. 10a-10 d, is to bend backone or both of the edges of the hafnium plates inwards toward thecentral part of the absorber part.

Since the absorber part is of hafnium and the top piece and the bottompiece are of stainless steel, they cannot be welded to each other, whichwould be the case if they were of the same material. In the following, afew proposals for possible attachments will be given. In the event ofreactor scram (fast insertion of the control rods), the stress is greaton the attachment between the absorber part and the bottom piece. Theattachment between the absorber part and the top piece, on the otherhand, is exposed to lower stress and can therefore be made incombination with the distance buttons 1 b (see FIG. 11a). This gives thedistance buttons a twofold function, namely, as spacers between thecontrol rod and the fuel, and as a large rivet for attachment betweenthe absorber part and the top piece. As an alternative to using thedistance button as attachment between the absorber part and the toppiece, a rivet joint can be used (see FIG. 11b).

The attachment with rivets 22 can be made in a plurality of differentways. FIGS. 12a-12 c show different alternatives. FIG. 12a shows how thestainless part 23 surrounds one end of a tray 24 of hafnium and theother end of the tray is inserted between the hafnium sheets 25 in theabsorber blades. The hafnium tray is welded to the hafnium sheets in theabsorber blades. The stainless part and the hafnium tray are weldedtogether by means of one or more rivets 22. In FIG. 12b, the lower partsof the hafnium sheets 25 in the absorber blades are compressed to makecontact with each other and surrounded by the stainless part 26. Thestainless part 26 and the hafnium sheets 25 are joined together by meansof one or more rivets 22. In FIG. 12c, one end of the stainless part isformed as a lug 27. The lug is inserted between the hafnium sheets 25 inthe absorber blades and is fixed by one or more rivets 22.

FIG. 13 shows a rivet joint between the absorber part and the bottompiece. The attachment can be made using any of the alternatives shown inFIGS. 12a-12 c. The advantage of a rivet joint is that it has a lowweight and is simple to mount. A disadvantage of a rivet joint is thatit contains many small parts which may possibly loosen during operation.An alternative to rivet joints is a locking-pin joint, which is shown inFIGS. 14a and 14 b. The absorber part terminates in a lug 28 of hafniumwhich is locked in the bottom part by means of a pin 29. FIG. 14b showsin a section E—E how the hafnium lug is inserted into and welded betweenthe hafnium sheets in the absorber part. A disadvantage of a locking-pinjoint is, among other things, that the control rod becomes heavy becauseof the considerable weight of the hafnium lug. A large quantity ofhafnium in the lower part of the control rod results in an inefficientuse of hafnium, since the major burnup takes place in the upper part ofthe control rod.

The absorber material in a control rod according to the invention may,for example, consist of pure hafnium or of some hafnium alloy, forexample a hafnium-zirconium alloy. Another common absorber alloy is analloy of cadmium, indium and silver, usually with a composition suchthat the nuclear properties of hafnium are imitated. The moderatorconsists of water or a solid moderator, for example zirconium hydride.

During the manufacture of a hafnium sheet, it is given a directedtexture. Upon neutron irradiation of the sheet, an irradiation growthtakes place, substantially in the direction of the texture. A controlrod made of hafnium sheet, as the one described above, thus runs therisk of being bent when subjected to neutron irradiation. One way ofpreventing this bending is to β-quench the hafnium sheet in a manneranalogous to that of a zirconium alloy. Such a method is described, forexample, in Swedish patent specification 7502865-4. The method comprisesheating the hafnium sheet up to such a high temperature that its textureis transformed from α-phase to β-phase, which has a random texture. Tomaintain the crystal structure in the hafnium sheet, it must be rapidlycooled again. In a random texture, the growth upon irradiation takesplace equally in all directions and hence bending of the control rod isprevented.

For hafnium the transformation temperature to β-phase lies at about1800° C. The transformation temperature for a zirconium alloy lies atabout 900° C. By alloying hafnium with some suitable metal, for exampleFe, Ni, Cr, Nb, the transformation temperature may be reduced to atransformation temperature which lies near the transformationtemperature of the zirconium alloy. When choosing a suitable alloy, itis important to ensure that the corrosion properties and hydrogenpick-up properties of hafnium are retained and preferably improved.After the β-quenching it is suitable to heat-treat the hafnium sheet torelieve stresses in the sheet caused by the β-quenching, so-calledstress-relieve annealing.

What is claimed is:
 1. A control rod for a boiling water reactor whichcontrol rod is submerged under water during operation, said control rodcomprising: a top piece; a bottom piece; and an absorber part fixedbetween said top piece and said bottom piece, said absorber partincluding neutron absorbing metal sheets arranged opposite each otherforming a cruciform cross-section comprising a central part and fourwing parts; a plurality of indentations being distributed intermittentlyin the longitudinal direction on each said metal sheet, a connectionbeing formed between corresponding indentations on opposite metal sheetsin order to space said metal sheets apart; and said central part andsaid wing parts being hollow and containing no other support structureor neutron absorbing material except for said connections, said centralpart and said wing parts extending between said top piece and saidbottom piece and being open at least at their top and bottom so as tofill the interior of said absorber part with water when submergedwhereby a water volume is surrounded by said metal sheets.
 2. A controlrod for a boiling water reactor which control rod is submerged underwater during operation, said control rod comprising: a top piece; abottom piece; and an absorber part fixed between said top piece and saidbottom piece, said absorber part including neutron absorbing metalsheets arranged opposite each other forming a cruciform cross-sectioncomprising a central part and four wing parts; said central part andsaid wing parts being hollow and containing no other support structureor neutron absorbing material, said central part and said wing partsextending between said top piece and said bottom piece and being open atleast at their top and bottom so as to fill the interior of saidabsorber part with water when submerged whereby a water volume issurrounded by said metal sheets.
 3. A control rod according to claim 1,wherein said neutron-absorbing material in the absorber part compriseshafnium.
 4. A control rod according to claim 2, wherein saidneutron-absorbing material in the absorber part comprises hafnium.
 5. Acontrol rod according to claim 1 wherein said metal sheets are L-shapedmetal sheets.
 6. A control rod according to claim 2 wherein said metalsheets are L-shaped metal sheets.
 7. A control rod according to claim 1,wherein the thickness of the metal sheets varies.
 8. A control rodaccording to claim 2, wherein the thickness of the metal sheets varies.